All of the above
Depends on what you want in an amp simulator
Most would just like a good cabinet simulator.
For SS - GK and MarkBass, have them.
For tube Peavey Revalver is a blast.
Cheap is Voxengo Boogex it's still very good - and it will load in cabinet impulses
many choices, many of the guitar amp modelers include bass amps.
http://www.guitarampmodeling.com/ is a good place to go to get some ideas.